Flexible transparent electrodes of silver nanowires sintered with metal oxide nanoparticles

Abstract

Disclosed are transparent electrodes made from a polymer matrix having semi-embedded therein a sintered silver nanowire composite material comprising silver nanowires and metal oxide nanoparticles.

Description

[0001] This international patent application claims the benefit of U.S. Provisional Patent Application No. 63 / 154,914, filed on March 1, 2021, which is incorporated herein by reference in its entirety for all purposes. Technical Field

[0002] Disclosed are electrodes of metal nanowires sintered with metal oxide nanoparticles, methods related thereto, and devices containing the electrodes.

[0003] background

[0004] Flexible transparent electrodes are indispensable components in various emerging flexible optoelectronic devices such as photovoltaic elements (PV), light-emitting diodes (LEDs), photodetectors, sensors and touch screen panels. Conventionally, iridium-doped tin oxide (ITO) is widely used in most electrical / optical devices due to its excellent conductivity and transparency. However, ITO films are manufactured by expensive deposition techniques equipped with ultra-high vacuum systems and high energy consumption. Even the indium of ITO is a rare and expensive material, which leads to high manufacturing costs for all applications. In addition, it has mechanical brittleness, making it a huge challenge for integration with emerging flexible devices. It is highly desirable to develop new flexible transparent alternatives to ITO electrodes.

[0005] Among many promising flexible transparent electrodes, silver nanowire (Ag NW) electrodes have been shown to be a potential candidate to replace conventional ITO electrodes. This is because they not only have excellent conductivity and transparency, but also possess mechanical flexibility described as ductile. Furthermore, Ag NW electrodes can be realized through very simple, low-cost, and easily scalable solution processing methods such as spin coating, Mayer rod coating, doctor blade coating, deep coating, and air spraying of nanowire dispersions. SUMMARY OF THE INVENTION

[0007] A simplified summary of the present invention is provided below to provide a basic understanding of some aspects of the present invention. This summary is not an extensive overview of the present invention. It is not intended to identify key or important elements of the present invention, nor is it intended to delineate the scope of the present invention. Instead, the sole purpose of this summary is to present some concepts of the present invention in a simplified form as a prelude to the more detailed description presented below.

[0008] Disclosed herein are transparent electrodes comprising a polymer matrix having semi-embedded therein a sintered silver nanowire composite material comprising silver nanowires and metal oxide nanoparticles.

[0009] In some embodiments, the metal oxide nanoparticles include at least one of sodium borohydride particles, lithium aluminum hydride particles, zinc oxide particles, titanium oxide particles, tin oxide particles, nickel oxide particles, and diisobutylaluminum hydride particles.

[0010] In some embodiments, the metal oxide nanoparticles include p-type metal oxide nanoparticles.

[0011] In some embodiments, the metal oxide nanoparticles include n-type metal oxide nanoparticles.

[0012] In some embodiments, the metal oxide nanoparticles have a high refractive index.

[0013] In some embodiments, the polymer matrix comprises at least one of polyimide, polydimethylsiloxane, and polyvinylpyrrolidone.

[0014] In some embodiments, the polymer matrix has a smoother surface roughness than another polymer matrix having an unsintered silver nanowire composite material comprising silver nanowires and the same metal oxide nanoparticles semi-embedded therein.

[0015] In some embodiments, the polymer matrix is ​​flexible.

[0016] Also disclosed is a foldable communication device comprising the transparent electrode disclosed herein. Also disclosed is a method for manufacturing the transparent electrode, the method comprising sintering a composite material comprising silver nanowires and metal oxide nanoparticles; and semi-embedding the sintered composite material in a polymer matrix.

[0017] In some embodiments, the sintering includes UV ozone surface treatment.

[0018] In some embodiments, the sintering is performed for 1 minute to 100 minutes.

[0019] In some embodiments, the sintering is performed for 10 minutes to 30 minutes.

[0020] In some embodiments, the sintering is performed after wetting with a sodium borohydride solution.

[0021] To achieve the foregoing and related ends, the present invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and accompanying drawings set forth in detail certain illustrative aspects and implementations of the invention. However, these are merely indicative of a few of the various ways in which the principles of the invention may be employed. Other objects, advantages, and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

[0022] Summary of the Figures

[0023] Figure 1 A table disclosing the differences between AgNW:PEDOT and AgNW:Oxide is depicted.

[0024] Figure 2 (a) and Figure 2 (b) SEM top view images showing bare Ag NWs dispersed on a glass substrate with and without chemical treatment, respectively. Figure 2 (c) and Figure 2 (d) SEM top view images showing treated and untreated ZnO NP films, respectively. Figure 2 (e) and Figure 2 (f) TEM images of treated and untreated Ag NW:ZnO NFS composites, respectively. Figure 2 (e) and Figure 2 The insets in (f) zoom in on the region of the cross-intersections between AgNWs, respectively.

[0025] Figure 3 Table 1 is depicted, which reports an overview of the average reduction in sheet resistance.

[0026] Figure 4 Depicted are AFM images showing the ZnO NF films before and after general chemical treatment ( Figure 4 (a)) and ZnO NF film after general chemical treatment ( Figure 4 (b) The untreated ZnO NF film showed RMS roughness and peak-to-valley roughness of 4.86 nm and 42.32 nm, respectively. In comparison, the treated ZnO NF film exhibited significantly reduced RMS roughness and peak-to-valley roughness of 1.52 nm and 12.81 nm, respectively.

[0027] Figure 5 Depicted are the XFS spectra of Ag 3d for Ag NW electrode composites with ZnO NPs treated and untreated by chemical treatment. 5 / 2 and Ag 3d 3 / 2 The peaks shifted from 367.1 and 373.1 eV to higher binding energies of 367.6 and 373.6 eV, respectively. Detailed Description of the Invention

[0029] However, the Ag NW electrodes directly dispersed on flexible substrates have shown stability problems, which are caused by their weak adhesion to the substrate and will easily fall off under mechanical operations (such as bending, stretching, etc.). Recently, the new technology of embedding Ag NW electrodes into polymer matrices has attracted attention as a new technology that successfully improves adhesion. The resulting embedded Ag NW electrodes show significantly improved mechanical stability to a variety of mechanical stresses / strains. However, due to the fact that most of the individual Ag NWs are buried by the polymer matrix, the charge transport between them and the adjacent layers is extremely limited to a very narrow channel on the top surface of the embedded Ag NW electrode. In addition, it is worth pointing out that there are still loose chemical contacts at the intersections between the Ag NWs, and there are still incoherent insulating polyvinyl pyrrolidone (PVP) ligand residues on the 11001 surface of the Ag NW. Whenever under mechanical operation, these will still uncontrollably change their electrical properties. In addition, the Ag NW electrodes are expected to lose conductivity in the extended time due to their chemical instability. Such instability is caused by the significant impact of atmospheric corrosion on Ag NW electrodes. It is important to note that the application of flexible devices is also subject to frequent mechanical operations with continuous electrical bias under humid conditions, yet there are few reports on the multi-operation stability of flexible Ag NW electrodes to date. Therefore, it is highly desirable to strategically develop passivated semi-embedded Ag NWs in polymer matrices by chemically stable materials, which simultaneously (1) provide better carrier transport channels with wide direct contact surfaces, (2) eliminate incoherent PVP ligands from Ag NWs, (3) enhance chemical bonding between Ag NWs, and (4) protect the passivating materials from moisture-induced degradation to achieve flexible AgNW electrodes semi-embedded in polymer matrices that are durable to multiple operations.

[0030] Compared to Ag NW:FEDOT-type nanowires, the present disclosure has different features, particularly robust stability of flexible Ag NW:oxide composite electrodes that are durable to multiple operations, such as simultaneous repetitive mechanical bending, continuous electrical biasing, and exposure to moisture conditions. Although both FEDOT and oxides are widely adopted materials in optoelectronic devices, the development of both types of Ag NW-based electrodes with FEDOT and oxides is important for influencing and advancing the development of flexible optoelectronic components.

[0031] Reference Figure 1 The differences between Ag NW:PEDOT and AgNW:oxide were compared.

[0032] Overall, Ag NW:PEDOT appears more useful for applications requiring high surface smoothness and high-quality electrical / optical properties (e.g., LEDs), while Ag NW:oxide is generally more useful for touch screen panels and foldable e-books, which are frequently used under harsh mechanical, electrical, and / or chemical conditions. Furthermore, Ag NW:oxide is generally useful for multilayer device applications due to its both hole and electron injection capabilities.

[0033] As described herein, a new class of highly durable flexible transparent electrodes comprises Ag NW composites sintered together with different types of metal oxide nanoparticles by a general solution method, which is then semi-embedded in a polymer matrix. The method of manufacturing a metal network with excellent electrical conductivity and optical transparency, which can be used as, for example, a transparent electrode. Such transparent electrodes can be used in many applications, including but not limited to display panels, organic light-emitting diodes, organic solar cells, and organic photodetectors.

[0034] As for the general solution method, we propose sodium borohydride, lithium aluminum hydride, and diisobutylaluminum hydride. Interestingly, this solution method is universally applicable to both n-type and p-type metal oxide nanoparticles in the composite electrode. The contributions of the universal chemical method to the different composite electrodes are (i) the removal of incoherently resistive capping ligands (such as PVP) around the Ag NW; (ii) the pinhole-free surface properties of the various metal oxide nanoparticle films by merging the nanoparticles together; and (iii) sintering between the Ag nanowires and the different metal oxide nanoparticles via the formation of coordination bonds, respectively. In addition, by semi-embedding the composite electrode in a polymer matrix, efficient charge transport can be ensured across the entire conductive surface and adjacent functional layers in multilayer optoelectronic devices (such as PV and LEDs). In addition, the optical properties of the composite can be tuned / improved by exploiting the high refractive index of the metal oxide nanoparticles. In this regard, we can easily generate interfaces with high refractive index differences.

[0035] Advantageous features of the electrodes described herein include one or more of the following:

[0036] 1. Simple solution processing and cost effectiveness: There are many different solution processing techniques that are capable of producing high-quality films to achieve the desired coating using simpler and less expensive equipment compared to complex and expensive evaporation systems.

[0037] 2. Green process: room temperature, indoor atmosphere and non-toxic process.

[0038] 3. It is conducive to large-area processing.

[0039] 4. Universally applicable process to form transparent flexible electrodes with various metal oxides: The entire fabrication process including chemical treatment is universally applicable to both n-type and p-type metal oxides as composite electrodes with AgNWs.

[0040] 5. Robust multi-operation stability: Enhanced stability to multiple mechanical flexibility tests simultaneously under exposure to moisture conditions accompanied by continuous electrical bias.

[0041] 6. Wide carrier transport channel with adjacent layers.

[0042] The disclosed hybrid Ag nanowires with metal oxide nanoparticles semi-embedded in a polymer matrix via a general solution process at least partially solve one or more of the following problems for hybrid electrode systems:

[0043] 1. The resistance problem at the intersections between Ag NWs caused by the insulating PVP firmly attached along the bottom surface of Ag NW at the nanoscale.

[0044] 2. Small pores of metal oxide nanoparticles in the membrane.

[0045] 3. Inert interaction between Ag nanowires and metal oxide nanoparticles.

[0046] The present disclosure at least partially solves / addresses the above problems by at least one of the following:

[0047] 1. Chemical Ag nanowire treatment removes the insulating PVP on the surface of the Ag NW, thus reducing the resistance between the Ag nanowires.

[0048] 2. Through chemical treatment, the small pores of the metal oxide nanoparticles in the film disappear due to the merging and rearrangement between the nanoparticles.

[0049] 3. Generate strong coordination bonds between AgNWs and metal oxide nanoparticles.

[0050] Therefore, these three achievements in this composite material by a general solution approach contribute to the durable stability to the operation of multiple loads (electrical bias, bending and high humidity).

[0051] Due to the solution process having a typical processing time of 30-300 seconds, in some embodiments, there is a problem of changing the bottom layer in embodiments in which a bottom layer is used. In such embodiments, there is a limited use of delicate materials in liquid alcohol-water based solutions. However, in some embodiments, the fabrication of the device is intended to be an anhydrous or substantially anhydrous process to eliminate / minimize moisture / water-induced changes in the underlying layered device structure.

[0052] The formation of flexible composite electrodes semi-embedded in a polymer matrix is ​​accomplished entirely in a solution process. First, Ag NWs are dispersed (e.g., by spin coating) on ​​a rigid support substrate with a smooth surface roughness (e.g., silicon wafer, glass, etc.) after UV-ozone (UVO) surface treatment of the substrate. Typically, UV light has at least one of two types of wavelengths: 185 nm and 254 nm. On top of the freshly prepared Ag NW electrodes, the incorporation of different types of metal oxide nanoparticles has been investigated by simple solution processes (e.g., spin coating).

[0053] In the following, zinc oxide nanoparticles (ZnO NFS) are used as a representative example for further discussion. Thereafter, the chemical treatment on the hybrid Ag NW:ZnO NFS provides three unique effects simultaneously: (i) removal of PVP ligands from the Ag nanowires; (ii) pinhole-free surface morphology of the ZnO NFS in the film; and (iii) generation of coordination bonds between the Ag NW and the ZnO NPs. It is important to note that this chemical treatment can be generally applied to many different hybrid Ag NW electrode systems with n-type or p-type metal oxide nanoparticles, such as ZnO, titanium oxide (TiO2), and tin oxide (SnO2) for n-type, and nickel oxide (NiO2) for p-type. x After this chemical treatment, the composite electrode is embedded in a polymer matrix such as colorless polyimide (CPI), polydimethylsiloxane (PDMS), etc.

[0054] Figure 2 (a) shows a top-view scanning electron microscope (SEM) image of a Ag NW electrode on a glass substrate after chemical treatment. It can be clearly seen that there is no residual PVP ligand around the surface of the Ag NW, however, Figure 2 As shown in (b), the disjointed PVP ligands are still firmly attached to the surface of the untreated Ag NW.

[0055] like Figure 3 As shown in Table 1, after the universal chemical treatment, an average decrease in the sheet resistance of 10 samples with respect to the untreated bare Ag NW electrode was confirmed, which was mainly due to the direct physical contact between the Ag NWs by removing the PVP ligands.

[0056] At the same time, if Figure 2 As shown in FIG d, it has been observed that the ZnO NP film formed on the UVO treated rigid substrate (such as silicon wafer, glass, etc.) has many small holes with a width of up to 75 nm between the ZnO NP clusters. In contrast, with chemical treatment, these small holes disappear and the ZnO NP film shows as Figure 2(c) shows the surface and continuous morphology of the SEM image. Atomic force microscopy (AFM) measurements of the untreated and treated ZnO NP films were compared with those of Figure 4 The SEM results explained in (a) and 4(b) are consistent. Through this treatment, the root mean square (RMS) roughness is significantly reduced from 4.86 nm to 1.52 nm, and the peak-to-valley roughness is significantly reduced from 42.3 nm to 12.8 nm.

[0057] Regarding the untreated Ag NW electrode composite, poor contact and no chemical interaction between the Ag NW and ZnO NFs were shown. Figure 2 This is confirmed by the TEM image in (f), which shows that the individual ZnO NPs weakly interact with the entire Ag NW as well as at the intersections between the Ag NWs. Figure 2 As shown in the TEM image (e), after this chemical treatment, there are enhanced interactions between ZnO NPs and Ag NWs not only along the Ag NWs but also at the intersections. These attractive forces can be explained by Figure 5 The new chemical coordination bond between Ag NW and ZnO NFs was confirmed by X-ray photoelectron spectroscopy (XPS) in the nanostructured Ag composite, which showed that the Ag3d 5 / 2 Positive binding energy shift of the core electron.

[0058] In one embodiment, the details of the process to fabricate flexible Ag NW electrode composite sintered with metal oxide nanoparticles follows the following chemical treatment.

[0059] Ag NW electrodes and subsequently metal oxide nanoparticles were deposited on a rigid support substrate and treated with UVO for 20 minutes. For the chemical treatment, a 0.5 M sodium borohydride solution dissolved in an 8:2 ratio of DI water:ethanol was used. First, the surface of the freshly prepared mixed Ag NW:metal oxide nanoparticles was wetted with the prepared sodium borohydride solution for an extended period of 30-300 seconds. Subsequently, various methods such as spin coating, Meyer rods, and blow-off methods were able to remove the solution from the surface.

[0060] What is achievable through the disclosure herein is a Ag NW electrode at least semi-embedded in a polymer matrix that is durable to multiple operating environments, such as simultaneous repetitive mechanical bending, continuous electrical bias, and exposure to moisture conditions. There are almost no reported studies on the stability of flexible electrodes under multiple operations.

[0061] Although the examples provided herein describe silver nanowires, this is for illustrative purposes only. The metal nanowires can be silver, gold, aluminum, platinum, palladium, or alloys thereof, although embodiments are not limited thereto. The metal oxide nanoparticles can be the same or different metal as the metal nanowires, but in most embodiments are different.

[0062] The metal oxide nanoparticles have a size suitable for forming strong bonds with the metal nanowires. In one embodiment, the metal oxide nanoparticles have a size wherein at least about 95% by weight of the particles have a size between about 1 nm and about 500 nm. In this context, size refers to the average cross-section of the particles, such as a diameter. In another embodiment, the metal oxide nanoparticles have a size wherein at least about 95% by weight of the particles have a size between about 5 nm and about 250 nm. In yet another embodiment, the metal oxide nanoparticles have a size wherein at least about 95% by weight of the particles have a size between about 10 nm and about 100 nm. In another embodiment, approximately 100% by weight of the metal oxide nanoparticles have any of the aforementioned sizes.

[0063] Embodiments also relate to methods of making a network of metal nanowires having metal nanoparticles present at the intersections where the nanowires intersect, and networks of metal nanowires made by such methods. The metal nanowire networks of the present invention can have metal nanoparticles within the network at the intersections where the nanowires intersect.

[0064] Unless otherwise indicated in the examples and elsewhere in the specification and claims, all parts and percentages are by weight, all temperatures are in degrees Celsius, and pressures are at or near atmospheric.

[0065] For any number or numerical range for a given property, a number or parameter from one range can be combined with another number or parameter from a different range for the same property to produce a numerical range.

[0066] Other than in the operating examples, or where otherwise indicated, all numbers, values ​​and / or expressions referring to quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about."

[0067] Although the present invention has been explained with respect to certain embodiments, it is to be understood that various modifications thereof will become apparent to those skilled in the art after reading this specification. Therefore, it is to be understood that the invention disclosed herein is intended to cover such modifications as fall within the scope of the appended claims.

Claims

1. A method for manufacturing a transparent electrode, comprising: Wetting the composite material comprising silver nanowires and metal oxide nanoparticles with a sodium borohydride, lithium aluminum hydride, or diisobutylaluminum hydride solution and then sintering; and The sintered composite material is semi-embedded in the polymer matrix.

2. The method according to claim 1, wherein the sintering comprises ultraviolet ozone surface treatment. The method according to claim 1 , wherein the sintering is performed for 1 minute to 100 minutes. The method according to claim 3 , wherein the sintering is performed for 10 to 30 minutes. 5 . The method according to claim 1 , wherein the metal oxide nanoparticles include at least one of zinc oxide particles, titanium oxide particles, tin oxide particles, and nickel oxide particles. The method of claim 1 , wherein the metal oxide nanoparticles comprise p-type metal oxide nanoparticles. The method of claim 1 , wherein the metal oxide nanoparticles comprise n-type metal oxide nanoparticles.

8. The method of claim 1, wherein the polymer matrix comprises at least one of polyimide, polydimethylsiloxane, and polyvinylpyrrolidone.

9. The method of claim 1, wherein the polymer matrix is ​​flexible.

10. A transparent electrode, characterized in that: Prepared by the method according to any one of claims 1 to 9.

11. A foldable communication device comprising the transparent electrode according to claim 10.

Images